The characterization of a sodium-dependent high affinity choline uptake system unassociated with acetylcholine biosynthesis

Abstract

1. 1. The cardiac ganglion of the horseshoe crab, Limulus polyphemus, was incubated in Chao's solution containing 0.01 μM [ 3H]choline at room temperature (25 ± 2°C) and the ganglion readily accumulated the radiolabel. 2. 2. The ganglion uptake of [ 3H]choline was linear over 60 min. 3. 3. Kinetic analysis revealed dual choline uptake systems within the cardiac ganglion, a high affinity uptake system ( K m = 2.2 μM, V max = 0.16pmoles/mg/min) and a low affinity system ( K m = 92.3 μM, V max = 3.08 prnoles/mg/min). 4. 4. The high affinity uptake system was sodium-dependent and inhibited by micromolar concentrations of hemicholinium-3. 5. 5. A 15 min pre-exposure of the ganglion to Chao's solution containing 90 mM potassium stimulated a significant increase in choline uptake. 6. 6. There was no detectable synthesis of [ 3H]acetylcholine from the [ 3H]choline taken up by the cardiac ganglion. The major portion of the extraetable label appeared in a fraction which co-electrophoresed with phosphorylcholine. 7. 7. These results suggest that the sodium-dependent high affinity [ 3H]choline uptake system of the cardiac ganglion subserves a specific requirement for choline which is unrelated to a cholinergic function.